Help! I keep getting billed for calls to the speaking clock....

Hi,
Help i keep getting billed for calls to the speaking clock. two per month.
I've contacted my land line service provider and searched the web for explanations.

- I have no phone connected to my land line, the only thing connected is a broadband router. Primus saver landline and broadband.
- I have no virgin box or sky box or anything else i can think of that might be making automatic calls. We have no alarm system in our flat itself.
- one call happened whilst i was on holiday everything was switched off except for the fridge freezer, boiler and battery operated room thermostat. No one else had access to the flat.
- I've been intouch with primus by email but their responses have dried up. They have refunded a number of the calls.
- When the bt engineer conected out land line he said that some one had taken our designated slot and he'd had to install a new one.

So I'm really stuck and hoping someone can suggest what I should do next. I guess a bt engineer needs to come out and check the line for faults external to our flat???

And could it be anything in our flat making the calls?

Your advice is much appreciated

Julie